1. Field of the Invention
The present invention generally relates to a sensor IC, and more particularly, to a sensor IC such as a magnetic sensor IC integrated with a magnetic sensor and additional circuits, and a pressure sensor IC integrated with a pressure sensor and additional circuits.
2. Description of the Related Art
FIG. 1 shows a schematic diagram of a prior art magnetic sensor IC. The magnetic sensor IC 21 is constructed with a single Hall sensor Hs, a single differential amplifier DAV, and load resistors Rc1, Rc2, all of which are formed on the same semiconductor chip.
The Hall sensor Hs is provided with the power source voltage Vcc, and is operative with a constant voltage. A sensor output voltage produced from an output terminal of the Hall sensor Hs is given by a sum of an unbalanced voltage V.sub.Hoff in the Hall sensor Hs and a product (V.sub.HO.B) of sensitivity V.sub.HO of the Hall sensor Hs and magnetic flux density B of a magnetic field applied to the magnetic sensor IC 21.
The sensor output voltage of the Hall sensor Hs is provided to input terminals of the differential amplifier DAV. The differential amplifier DAV is constructed with transistors Q1, Q2, a current-feedback resistor R.sub.E, and constant current sources J1, J2. Output currents Ic1, Ic2 of the differential amplifier DAV are respectively converted to voltages by the load resistors Rc1, Rc2, and are produced as an output voltage Vout1 between output terminals T6, T7.
In the differential amplifier DAV, values of the load resistors Rc1, Rc2, the current-feedback resistor R.sub.E, and bias currents I1, I2 generated in the current sources J1, J2 are adjusted so that a voltage gain of the differential amplifier DAY could be a value A, where Rc1=Rc2, and I1=I2.
In this case, the output voltage Vout1 of the magnetic sensor IC 21 is given by the following equation (1). EQU Vout1=A.V.sub.HO.B+A.(V.sub.Hoff +V.sub.Aoff) (1)
where
A: voltage gain of the differential amplifier DAV, PA1 VHO: sensitivity of the Hall sensor Hs, PA1 B: magnetic flux density of a magnetic field applied to the magnetic sensor IC, PA1 V.sub.Hoff : an unbalanced voltage in the Hall sensor Hs, and PA1 V.sub.Aoff : an input offset voltage of the differential amplifier DAV.
In the equation (1), a first term indicates a Hall output voltage component when the magnetic flux density B is applied to the magnetic sensor IC 21. The following equation (2), which is given by dividing the first term of the equation (1) by the magnetic flux density B, indicates sensitivity K1 of the magnetic sensor IC 21. EQU K1=A.V.sub.HO ( 2)
A second term of the equation (1) is given by multiplying the sum of the unbalanced voltage V.sub.Hoff and the input offset voltage V.sub.Aoff of the differential amplifier DAV by the voltage gain A of the differential amplifier DAV, and indicates an unbalanced voltage component.
In a process of manufacturing the above-discussed magnetic sensor IC, deviation of electrode size of the Hall sensor Hs, etc., may usually occur due to dispersion in a photo etching process and a diffusion process, etc. Whereby, the unbalanced voltage V.sub.Hoff of the Hall sensor Hs is dispersed. Further, due to the dispersion in the photo etching process and the diffusion process, etc., the input offset voltage V.sub.Aoff of the differential amplifier DAV is also dispersed.
In the prior art magnetic sensor IC shown in FIG. 1, an amount of dispersion of the unbalanced voltage V.sub.Hoff of the Hall sensor Hs and an amount of dispersion of the input offset voltage V.sub.Aoff of the differential amplifier DAV are respectively multiplied by the voltage gain A of the differential amplifier DAV. Therefore, the output voltage Vout1 is dispersed. Accordingly, in the prior art magnetic sensor IC, there is a problem that it is difficult to use for measuring a magnetic field with high precision.
Further, the large dispersion of the unbalanced voltage component may prevent a magnetic sensor IC of high-sensitivity from being produced.